Mathieu,You say that both the CDs you are trying open fine on your amd64 laptop. It sounds like you may be trying to use an AMD/Intel architecture installation disk in a Power PC architecture computer. That won't work.

You'll need to download and burn the iso file of the Power PC architecture of Debian 5. Here is the url to the page from which you can download the DVD:

However I could not get the MacMini to boot onto the debian installer. I tried the:- 'C' holding approach (does not work at all)- 'Options' holding approach (Only the hard drive icon shows up)- 'Option'+'Command'+'o+'f' opens a terminal: